--40 at Altabird. Survivable but not really skiable. 18in of fresh felt like air--just skiing on the frozen mank below. Lifts were empty, except the SB tram which was mobbed, but at the top freezing fog and unable to see a thing. Skied Chip's by Braille and quit after one run.
--40 Windchill at the top of the Aiguille du midi--coldest day of the year there. Once down out of the wind, though, it was pleasant.
Note that windchill is the equivalent still temperature on bare skin. Assuming you're not skiing naked, windchill exaggerates the cold.
I survived a bivouac on Mt Washington-- minus 30F that night, huddled behind a rock to get out of the 140mph max wind (steady 100mph), in a down parka.